How to use antialiasing with bgl drawn elements?
Turns out any kind of smoothing you want to use with GL_POLYGON_SMOOTH, GL_LINE_SMOOTH etc. (antialiasing simple shapes) requires GL_BLEND also be enabled. 

I discovered that if you're using GL_POLYGON with GL_POLYGON_SMOOTH enabled, you get weird breaks in your mesh shapes. For this reason it's best to leave GL_POLYGON_SMOOTH off, then surround the shape created with GL_POLYGON with a line from GL_POLYGON_LOOP and then enable GL_LINE_SMOOTH. This gives the appearance of antialiasing around the shape without the weird breaks (demo in code below).

import bpy,bgl


def draw_callback_px(self, context):

    #Set color to white.
    bgl.glColor4f(1, 1, 1, 1)

    #Using polygon smoothing alone creates
    #lines through shape... better to leave
    #off and just smooth outer line.
    #bgl.glEnable(bgl.GL_POLYGON_SMOOTH)

    #GL_BLEND must be enabled for any
    #smoothing to work.
    bgl.glEnable(bgl.GL_BLEND)
    bgl.glEnable(bgl.GL_LINE_SMOOTH)


    #Can't just stack GL_POLYGON
    #and GL_LINE_LOOP
    bgl.glBegin(bgl.GL_POLYGON)
    for x, y in self.mouse_path:
        bgl.glVertex2f(x, y)
    bgl.glEnd()

    bgl.glBegin(bgl.GL_LINE_LOOP)
    for x, y in self.mouse_path:
        bgl.glVertex2f(x, y)
    bgl.glEnd()


    bgl.glDisable(bgl.GL_BLEND)
    bgl.glDisable(bgl.GL_LINE_SMOOTH)
    #bgl.glDisable(bgl.GL_POLYGON_SMOOTH)   



class ModalDrawOperator(bpy.types.Operator):
    bl_idname = "object.modal_operator"
    bl_label = "Simple Modal Operator"

    def modal(self, context, event):
        context.area.tag_redraw()

        if event.type == 'LEFTMOUSE':
            self.mouse_path.append(
            (event.mouse_region_x, event.mouse_region_y))

        if event.type in ['RIGHTMOUSE','ESC']:
            context.space_data.draw_handler_remove(self._handle, 'WINDOW')
            return {'FINISHED'}

        return {'RUNNING_MODAL'}


    def invoke(self, context, event):
        self.mouse_path = []

        args = (self, context)
        self._handle = context.space_data.draw_handler_add(
        draw_callback_px, args, 'WINDOW', 'POST_PIXEL')

        context.window_manager.modal_handler_add(self)
        return {'RUNNING_MODAL'}


bpy.utils.register_class(ModalDrawOperator)
bpy.ops.object.modal_operator('INVOKE_DEFAULT')